feat(ai_trade): SIGNAL_TTL_SECONDS env 추가 (F5 part 2)

신호 expires_at 계산용 TTL (default 300s). 환경별로 조정 가능.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ai_trade/config.py

@@ -53,6 +53,9 @@ class Settings:
     min_momentum_for_buy: str = field(
         default_factory=lambda: os.getenv("MIN_MOMENTUM_FOR_BUY", "strong_up")
     )
+    signal_ttl_seconds: int = field(
+        default_factory=lambda: int(os.getenv("SIGNAL_TTL_SECONDS", "300"))
+    )
 
     @property
     def kis_is_virtual(self) -> bool: